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CASE STUDIES. Methane, Ethane, Butane,... Analysis shows * composition of C and H only. Conclusion: the molecules are Hydrocarbons: made of C and H only* They cannot take any more H's. Conclusion: molecules are Saturated Hydrocarbons, made using C-H single bondsMethyl bromide, ethyl Chloride, ... Conclusion: molecules are made using single bonds.
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1. Cpt 3. ALKANES & ALKYL HALIDES Objectives:
* Name and describe the structure and properties of alkanes
* Name and describe the structure and properties of alkyl halides
3. Introduction Definitions
* Alkanes = compounds of C & H bound with single bonds only.
Synonyms: * saturated hydrocarbons
* aliphatic compounds (associated with fats & oils)
Alkyl halides = compounds of C, H, and Halogens bound with single bonds only.
4. 3.1. Structure General Formulas:
a. Alkanes
*1. Open Chain Compounds
Formula: CnH2n+2
Types of chains:
* Straight (normal): all C's in one line
Ex: Pentane
* branched: C's branch off other nonterminal C's
Ex: isopentane
5. Alkanes (Continued) Isomers: compounds with same numbers & kinds of atoms and different bond arrangements
Constitutional isomers: differ in the way atoms are connected
ex: pentane & isopentane
Homologs: compounds differing only by one CH2 group
Ex: butane & pentane
6. Alkanes (Continued 2) Classes of C's:
* primary (1o): connected to only 1 C
* secondary (2o): 2 C’s
* Tertiary (3o): Connected to only 3 C’s
* quaternary(4o): four
Note: to qualify as 1o-3o, an alkyl halide must have the appropriate C-X bond
7. Classes of C's (Example)
8. Alkanes (Continued) *2. Cyclic alkanes:
General formula: CnH2n
Can be
Fully cyclic
Cyclic with side-chains
9. Cycloalkanes (Examples)
10. b. Alkyl halides General structure:
Open chain Alkyl Halides:
CnH(2n + 1)X. X = F, Cl, Br, I
Cyclic Alkyl Halides
CnH(2n-1)X
Types of C’s for Alkyl halides: primary, secondary, tertiary
11. Alkyl Halides (examples)
12. 3.2. Nomenclature Definition: Procedure to name compounds
Standard System: IUPAC (International Union for Pure and Applied Chemistry)
Parts of name: prefix-parent-suffix
Suffix for alkanes and alkyl halides: ane
13. a. Open Chain Alkanes (and alkyl Halides)
# of C's Parent Name
1 meth methane
2 eth ethane
3 prop propane
4 but butane
5 pent pentane
Following chains: greek roots. More: T3-2, pg 83
14. Open Chains (Continued) Branching Chains have:
* Parent (main) Chain: the longest, or the one with largest # of branching points. It gives name of compound
* Branch chain: substituent
Substituent name: replace suffix ane by yl name of hydrocarbon
# C's prefix name
1 meth methyl
3 prop propyl
10 dec decyl
15. Naming branching alkanes *Number the main chain C's using lowest set of numbers for the sub's. Numbers not needed: 3-C chains
* name subs in abc order (di, tri, tetra, ...not included)
* use di, tri, tetra, penta,...for repeats of subs
* Halogen sub’s names: fluoro, chloro, bromo, iodo.
16. Open Chain Alkanes (Examples)
17. Open Chain Alkanes (Examples names) A: 3-Ethyl-2,6-dimethyl-4-propyloctane
B:
18. * Complex substituents: Case of substituted substituents
Name the substituent: determined by the longest sub chain.
#1 position on sub: on the C that connects to the main chain
Naming rules: same as for main chain
19. Complex substituents (Examples)
20. * Alkyl Substituents with special names
21. b. Cycloalkanes. Thought teaser: What must be done to build cyclopentane from pentane?
Definition: Cycloalkanes = Saturated cyclic hydrocarbons
Synonym: alicyclic compounds
General formula: CnH2n
Nomenclature: place the prefix cyclo before name of corresponding alkane.
#of C's Name
Alkane Cycloalkane
5 Pentane Cyclopentane
22. Cycloalkanes (Continued) Substituents: must have less C’s than cycle
* must have lowest set of #'s
* are name in abc order
* #1 = 1st sub in abc order
Cycle as Substituent: when Side-chain has more C's than cycle and carries the main name
ex:
* 2-(1-methylcyclopropyl)pentane
* 4-(3-isopropyl-4-methylcyclopentyl)-3-t-butylhexane
23. 3.3. Conformational analysis Definition: Study of energy effects on bond arrangements
Basic principle: Atoms rotate around single bonds
Conformation: arrangement of atoms in a molecule after rotation around a single bond
Conformer (conformational isomer): structure obtained from a conformational change.
24. Conformational Analysis (Continued) Dihedral angle: between two substituents on two adjacent C's, looking through the C-C bond.
Sawhorse Representation: oblique structure representation used to show conformations
Newman Projections: view of sawhorse structure with one carbon behind the other.
Staggered Conformation: most stable. Atoms on neighboring C's are as far away from each other as possible. Dihedral angle: 60 deg. Van Der Waals interactions: minimum
Eclipsed conformation: least stable. Atoms on different C's: as close to one another as possible. DA = 0 dg. Maximum VDW repulsion.
25. Conformational Analysis (Continued 2) Van der Waals repulsions: take place btw atoms when they are too close.
Tortional Strain: energy barrier against rotation through eclipsing conformations.
Steric Strain: VDW repulsion between bulky group which are too close to one another.
Conformation energy plot: based on changes in steric & tortional strain
26. a. Analysis of Open chain compounds Ex2: Butane
conformation groups involved Energy level (kcal/mol)
Anti (staggered) CH3’s farthest apart baseline
Eclipses 2 x H-CH3 3.8
H-H 1.0
Gauche (staggered) CH3-CH3 DA: 60o 0.9
Eclipses CH3-CH3 3.8
2 x H-H 2 x 1.0
Gauche (staggered) CH3-CH3 DA: 60o 0.9
Eclipses 2 x H-CH3 3.8
H-H 1.0
27. Conformationn energy plot Shows relation between conformations and energy levels.
Based on changes in steric & tortional strain
Ex: butane
extra ex: Do conformational analysis and energy plots for
* 2-methylbutane(examined through c2-c3 bond)
28. b. Cyclic Compounds Thought teaser
* Cyclopropane: Compare C-C-C angles to the ideal tetrahedral angle
Angle strain: bond distorsion due to difference in C-C-C angle in a cycle compared to the normal 109o tetrahedral angle
Bent bonds: sgm bonds formed from nonaligned orbitals
29. Cycloalkanes Strain Energy cycle angle angle Strain strain energy
cyclopropane 60 49 27.6
cyclobutane 90 19 26.4
cyclopentane 108 1 6.5
cyclohexane 111 2 0
Puckered ring conformations: adopted by rings to minimize angle, torsional and/or steric strains.
30. Cycloalkanes (Examples)
31. *1. Chair Conformation of Cyclohexane Cycle is puckered in form of a chair
Chair = the most stable
Features:
* C-C-C angles: 109o.
* All dihedral angles: 60o.
* conformations: all staggered, either gauche or anti.
* Strains (all kinds) : minimum.
32. Cyclohexane Chair conformations (Continued) Positions of substituents
* Axial Position: vertically below or above ring
1,3-diaxial strain: steric strain between atoms in axial positions
* Equatorial position: slightly above or below horizontal plane of molecule
Conformation inversion (cycle flipping)
Axial position become equatorial and vice-versa
ex: 1-Ethyl-3-methylcyclohexane
33. Cyclohexane Chair conformations (Continued 2)
34. Stability of disubstituted cyclohexanes Thought teaser: Which isomer of 1,2-Dimethyl cyclohexane is most stable: equatorial-equatorial or equatorial-axial?
* 1,2-Disubstituted cycles
Most stable: Trans isomer
* 1,3-Disubstituted cycles
Most stable: Cis isomer
Ex: 1,3-Dimethyl cyclohexane
extra ex: 1,4-dimethylcyclohexane
35. *2 Boat Conformation of Cyclohexane. Observed:
* Strain of 7.0 kcal/mol;
* No angle strain;
* Eclipsing atoms at 2 C-C bonds
Types of strains:
* steric, between H's on C1 & C4
* tortional, due to eclipsing conformations
36. Twist-boat conformation Intermediate between chair and boat conformations
Adopted to relieve partially boat strain
ex: 1,4-ditertiobutylcyclohexane